In line with the trend for high strengthening and weight reductions in the automotive industry, along with recent increases in the prices of ferro alloys, the use of rare metal resources as a geopolitical bargaining chip has continued. Accordingly, there is a need to develop an ultra-fine grained wire rod having high strength and high ductility, even without the addition of ferro alloys.
Typically, techniques of adding alloying elements, such as niobium (Nb), titanium (Ti), and vanadium (V), have been suggested in order to refine a microstructure. These techniques may be regarded as a method of obtaining fine ferrite phases by allowing the added alloying elements to form precipitates and the precipitates to prevent the growth of austenite grains.
Also, in addition to the method detailed above, a controlled rolling technique, in which rolling is performed at a temperature appropriate for minimizing grain size, has also been used, and, as a result, a diameter of ferrite grains has been able to be refined to about 20 μm.
In addition to the controlled rolling technique, since there have been continuous requirements for grain refinement, a Thermomechanical Control Process (TMCP), in which mechanical properties are improved through the refinement of a grain structure by control of a rolling speed and a cooling rate, has also been introduced.
However, all of the foregoing grain refinement techniques may be suitable for producing steel plates, but may be difficult to apply to the production of wire rods. That is, since a wire rod may have a very high cross-section reduction rate in comparison to that of a thick steel plate, a rolling speed may be relatively fast, control of a cooling rate may not be facilitated, and in particular, with respect to water cooling, surface defects may be generated by the formation of martensite grains on the surface thereof. Therefore, the TMCP having a key role in controlling the rolling speed and the cooling rate may not be suitable for wire rods and a manufacturing technique suitable for wire rods has also been required.
Patents related to a technique of manufacturing a fine grained, high-strength, high-toughness wire rod include Japanese Patent Application Laid-Open Publication Nos. 2009-62574, 2009-138251, and 2009-132958. These patents may be limited to a method of obtaining a segmented ferrite and cementite structure through fractures in cementite grains by performing high reduction rolling, in addition to the addition of alloying elements, a cooling rate change, and controlling of alloying elements and cooling rates.
In the case of the segmented ferrite and cementite structure, there may be an advantage in that segmented ferrite grains act as fine precipitate hardening materials to significantly increase the strength of a wire rod, but there may also be disadvantages, in that a decrease in ductility may inevitably be accompanied by an increase in strength according to the Hall-Petch equation.
Also, since rolling must be performed after the formation of cementite grains or the like in order to segment cementite grains or the like, there may be a need for widening a two-phase region in which cementite grains or the like may be formed, and, for this purpose, the addition of alloying elements may be essential. Therefore, an increase in costs may also be incurred.
Other techniques related to a fine grained wire rod include an ultra-fine grained wire rod manufactured by using a powder metallurgy method using fine powder. However, a wire rod manufactured in a powder metallurgy scheme may have a limitation in its use and may have a disadvantage in that its strength may be decreased due to a decrease in sinterability of fine particles thereof during sintering.
Meanwhile, other techniques related to manufacturing an ultra-fine grained wire rod by using a rolling or cooling method may be relatively restrictive in number and most thereof may only be a technique limiting a size of a pearlite structure through controlling finish rolling and lead patenting (LP) temperatures. In particular, in consideration of the fact that the pearlite structure itself has fine grains, these techniques may not have a special technical meaning.
Therefore, a technique for manufacturing an ultra-fine grained wire rod by using carbon steel may have high utility. However, since a satisfactory technique may not have been suggested to date, there may be an urgent need for developing a technique therefor.